Real Estate and Accumulated Depreciation - Schedule III (Summary of Real Estate Investments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 405,000
Land	193,658
Building and Improvements	504,536
Costs Capitalized Subsequent to Acquisition	61,293
Gross Amount Carried	759,487	$ 671,210	$ 647,839	$ 635,447
Accumulated Depreciation	72,656	53,449	$ 35,982	$ 19,598
Acquired intangible assets	103,121	$ 103,285
Tax basis	718,100
Accumulated Amortization	(41,700)
421 W. 54th Street - Hit Factory (6)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Land	4,723
Building and Improvements	1,757
Costs Capitalized Subsequent to Acquisition	9
Gross Amount Carried	6,489
Accumulated Depreciation	247
400 E. 67th Street - Laurel Condominiums
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	44,610
Land	10,653
Building and Improvements	55,682
Costs Capitalized Subsequent to Acquisition	86
Gross Amount Carried	66,421
Accumulated Depreciation	7,456
200 Riverside Blvd - ICON Garage
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	5,390
Land	13,787
Building and Improvements	5,510
Costs Capitalized Subsequent to Acquisition	0
Gross Amount Carried	19,297
Accumulated Depreciation	723
9 Times Square
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	55,000
Land	54,153
Building and Improvements	76,454
Costs Capitalized Subsequent to Acquisition	25,205
Gross Amount Carried	155,812
Accumulated Depreciation	19,138
123 William Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	140,000
Land	50,064
Building and Improvements	182,917
Costs Capitalized Subsequent to Acquisition	30,717
Gross Amount Carried	263,698
Accumulated Depreciation	30,138
1140 Avenue of the Americas (5)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	99,000
Land	0
Building and Improvements	148,647
Costs Capitalized Subsequent to Acquisition	5,276
Gross Amount Carried	153,923
Accumulated Depreciation	14,429
8713 Fifth Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	10,000
Land	4,730
Building and Improvements	9,245
Costs Capitalized Subsequent to Acquisition	0
Gross Amount Carried	13,975
Accumulated Depreciation	272
196 Orchard Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	51,000
Land	55,548
Building and Improvements	24,324
Costs Capitalized Subsequent to Acquisition	0
Gross Amount Carried	79,872
Accumulated Depreciation	$ 253
Building
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Useful life	40 years
Land Improvements
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Useful life	15 years
Minimum | Furniture and Fixtures
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Useful life	5 years
Maximum | Furniture and Fixtures
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Useful life	7 years